Switching systems have traditionally used localized apparatus to switch voice or data from one line to the other. Such systems utilize space division switching, time division switching or combinations of the two.
In such systems there are often low data rate and capacity lines connected to the switching system as well as high data rate and capacity trunks, the switching system multiplexing the signals from the low data rate lines to the high data rate trunks. Such systems require synchronous operation of the system with the high data rate trunks and must be designed to accommodate the highest capacity and highest data rate to be expected.
Distributed systems have been proposed which accommodate different data rates. One such system is described in U.S. Pat. No. 4,679,191 issued July 7, 1987, assigned to CXC Corporation. In this system a synchronous time division multiplexed ring or loop is used in which messages are passed along the loop from various nodes. Each node receives digitized voice, or data signals sent at various different data rates.
In that system, data at each node is packetized and is transmitted around the loop, which operates as a LAN. While a loop system is described, it is clear that the LAN need not be in that configuration. Each node receives the incoming signal stream, buffers and packetizes it, and transmits it along the LAN to be received at another node. In the system described, one or another token ring protocol is used.
Each node must be synchronized to the ring and to the other nodes, and must contain both transmit and receive circuitry which is designed to transmit and receive packets at the LAN frequency. Each node receiver retransmits every signal not destined for itself. Therefore if the frequency of the LAN is to be increased, e.g. to accommodate higher data rate signals, every node connected to the LAN must be modified to handle the increased bandwidth. This imposes a significant cost penalty on those nodes which are connected to external apparatus which lower data rate data.
In addition, in the described architecture, since each node receives and repeats the ring data, each node contributes jitter to the signal passing along the LAN. Even inactive devices add to this jitter. Therefore the number of devices on the LAN is limited by the tolerance of the LAN to the jitter.
Further, high bit rate services require special jitter handling. For example, the FDDI LAN ring, which operates at 125 MHz has special jitter handling protocols.
The described CXC system clearly has significant limitations and is very costly.